Andrew-- Very cool. I've read about the "cone centraliseur" and would love to own a Darlot with one, but they usually go for big money. My guess is the cone was a bit of a marketing gimmick, since you don't need a cone to switch out the groups. I've also pretty much decided that the diffusion control on my Vitax doesn't really do much other than defocusing the lens. Wollensak just made a minor change from the Dallmeyer diffusion "feature". Dallmeyer switched the rear group around so the negative element was at the back of the lens and it could be unscrewed to "diffuse" the lens. If you read some of the articles on Dan's site, you can see not everyone was convinced of the utility of that "feature". The Vitax also has the negative element in the rear, but includes a radial drive to move the positive element of the rear group forward. The Vitax is an amazing lens, beautifully sharp in the center, but I'm not finding a lot of use for the diffusion control.
I'd love to see some photos with your Darlot's groups switched out. It sounds like the Petzvals have some similar properties to the rapid rectilinear lenses in that you can use a single group at about double the focal length, but somewhat uncorrected with a single group. Although since the Petzval isn't exactly symmetrical like the RR, it sounds like you got something on the order of quadrupling the focal length--from 6" to 24".
Joe-- That sounds like a nice classic whole plate Petzval. If the Voigtlander reputation is true, it should be an excellent lens.